152
R. Mattana et al.
heavy metals, semiconductors, insulators, topological insulators, etc.) and devices
are currently investigated. In this paragraph, we aim at discussing the spin-charge
conversion using topological insulators as well as the heat-spin current conversion
in ferromagnetic insulator/paramagnetic metal devices.
5.2.2.1 Spin-Charge Conversion
Conversion between charge and spin currents has been a very active branch of spintronics in the last couple of years. Such a conversion can be achieved in bulk materials
(the so-called spin Hall effect) or at interfaces (Edelstein–Rashba effect) relying on
the spin–orbit interaction and/or extrinsic effects. The studied materials are usually
NM metals with a large spin–orbit coupling. A second approach is to rely on spin–
orbit properties at the interfaces either at Rashba interfaces or through the surface
states of topological insulators (TIs). A complete review of this approach can be
found in J. Sinova et al. [44] or A. Soumyanarayanan et al. [36].
In the following, we provide an example where angle-resolved photoemission
spectroscopy (ARPES) measurements have allowed to understand the spin-charge
conversion from the α-Sn TI [see Fig. 5.14a]. In this spin-charge conversion study,
the spin current is generated through the magnetization dynamics induced at the
magnetization resonance of a Fe layer by an external rf field. The generated spin
current then diffuses to the α-Sn top surface. Injection of a spin current into a TI
induces a spin accumulation on one side of the Fermi contour of the Dirac cone as
well as a spin depletion on the other side [see Fig. 5.14b]. As a consequence, this spin
injection results in a charge current. Importantly, note that the spin-charge conversion
is not observed when Fe is deposited directly on α-Sn, but is observed when a thin Ag
layer is inserted at the interface [see Fig. 5.14c]. By performing ARPES measurement
to probe the DOS, it has been shown that the deposition of a sub-monolayer of Fe
on α-Sn indeed suppresses the Dirac cone, that is a signature of the TI, whereas it
is still observable after deposition of a Ag layer [see Fig. 5.14d]. Hence the absence
of spin-charge conversion at Fe/α-Sn interfaces is clearly ascribed to the loss of TI
surface states after the Fe deposition. This study shows that characterization of the
DOS by ARPES measurements is a very useful and unique technique to understand
spin-charge conversion at such spinorbitronic interfaces.
5.2.2.2 Heat-Spin Conversion
Similarly to the Seebeck effect, the spin Seebeck effect describes the generation of a
spin voltage from a temperature gradient in a FM conductor or insulator. Longitudinal
spin Seebeck effect (LSSE) refers to experiments where the spin current generated
is parallel to the temperature gradient. Materials used are FM materials (conducting
R. Mattana et al.
heavy metals, semiconductors, insulators, topological insulators, etc.) and devices
are currently investigated. In this paragraph, we aim at discussing the spin-charge
conversion using topological insulators as well as the heat-spin current conversion
in ferromagnetic insulator/paramagnetic metal devices.
5.2.2.1 Spin-Charge Conversion
Conversion between charge and spin currents has been a very active branch of spintronics in the last couple of years. Such a conversion can be achieved in bulk materials
(the so-called spin Hall effect) or at interfaces (Edelstein–Rashba effect) relying on
the spin–orbit interaction and/or extrinsic effects. The studied materials are usually
NM metals with a large spin–orbit coupling. A second approach is to rely on spin–
orbit properties at the interfaces either at Rashba interfaces or through the surface
states of topological insulators (TIs). A complete review of this approach can be
found in J. Sinova et al. [44] or A. Soumyanarayanan et al. [36].
In the following, we provide an example where angle-resolved photoemission
spectroscopy (ARPES) measurements have allowed to understand the spin-charge
conversion from the α-Sn TI [see Fig. 5.14a]. In this spin-charge conversion study,
the spin current is generated through the magnetization dynamics induced at the
magnetization resonance of a Fe layer by an external rf field. The generated spin
current then diffuses to the α-Sn top surface. Injection of a spin current into a TI
induces a spin accumulation on one side of the Fermi contour of the Dirac cone as
well as a spin depletion on the other side [see Fig. 5.14b]. As a consequence, this spin
injection results in a charge current. Importantly, note that the spin-charge conversion
is not observed when Fe is deposited directly on α-Sn, but is observed when a thin Ag
layer is inserted at the interface [see Fig. 5.14c]. By performing ARPES measurement
to probe the DOS, it has been shown that the deposition of a sub-monolayer of Fe
on α-Sn indeed suppresses the Dirac cone, that is a signature of the TI, whereas it
is still observable after deposition of a Ag layer [see Fig. 5.14d]. Hence the absence
of spin-charge conversion at Fe/α-Sn interfaces is clearly ascribed to the loss of TI
surface states after the Fe deposition. This study shows that characterization of the
DOS by ARPES measurements is a very useful and unique technique to understand
spin-charge conversion at such spinorbitronic interfaces.
5.2.2.2 Heat-Spin Conversion
Similarly to the Seebeck effect, the spin Seebeck effect describes the generation of a
spin voltage from a temperature gradient in a FM conductor or insulator. Longitudinal
spin Seebeck effect (LSSE) refers to experiments where the spin current generated
is parallel to the temperature gradient. Materials used are FM materials (conducting